Chemistry Department Colloquium

Photo-initiated reactions that form chemical bonds are important for the conversion and storage of solar energy. Iodide oxidation yields an I-I bond that is key to sensitizer regeneration in dye-sensitized solar cells. The detailed mechanism(s) by which iodide oxidation yields the I-I bonds present in I2-• and I3-remain speculative. Here direct evidence that metal-to-ligand charge transfer (MLCT) excited states can directly yield iodine atoms that subsequently react to form I-I chemical bonds, kinetic and thermodynamic data are summarized in the Eo = -0.82 RuII/+(bpz)2+
Scheme 1
Jablonski-type diagram shown in Scheme 1. The


Eo = +0.82
RuII(bpz-)+, I2

kcr = 2.1 ± 0.3 × 1010 M-1 s-1
compound [Ru(bpz)2(deeb)](PF6)2, where bpz is
- 2 I¬
RuII(bpz-)+, I3-+ I¬-
kI2 = 3.3 × 109 M-1 s-1
+ I2×
2,2-bipyrazine and deeb is 4,4’-(CO2Et)2-2,2’-bpy
= 1.75 s
V vs SCE
is abbreviated Ru(bpz)2+. Excited state electron
-
+ I--1
kI = 2.4 ± 0.2 × 1010 M-1 s
transfer to yield the iodine atom is favored by 430
Eo = +0.93 RuII(bpz-)+, I
mV. Reaction of the iodine atom with iodide to
+ I
¬
ket = 6.5 ± 0.3 × 1010 M-1 s-1 RuII*/+(bpz-)2+*
make an I-I bond lowers the free energy stored by 110 mV. Charge recombination to yield ground state products, Ru+ + I2-•  RuII + 2I- is downhill (-Go = 1.64 eV) and occurs with a rate constant of 2.1 x 1010 M-1 s-1, almost ten times larger than the I2-• disproportionation rate constant. Unwanted recombination to I2-• has been proposed to lower the efficiency of dye-sensitized solar cells and this data shows that it can be a very fast reaction [1].

Rapid photo-induced electron injection into TiO2 and regeneration by a donor, D, such as iodide or phenothiazine, yields sensitizers in an environment distinctly different from that prior to light absorption. Spectroelectrochemical measurements indicate that the injected electron influences the absorption spectr